Not Applicable
Silicon wafers for semiconductor production pass through many steps in transitioning from a slice of silicon to a collection of functional integrated circuits on a silicon wafer. Between many of these steps the wafers must be removed from a cassette, have their orientation changed, be placed in fixtures and be transported to the next step. All of these operations are performed in a clean room. One of the present technologies in wafer handling uses backside contact by vacuum grippers. This technology requires that there be a front side of the wafer and a backside with the backside subject to being handled by the vacuum grippers. A second technology in wafer handling grips the wafers by the edges but is subject to sudden exertions of force and mechanism wear that cause contamination. Both of these technologies have suffered from wafer jams when inserting and removing the wafers from the cassette, the wafer protrusion problemxe2x80x94misalignment of the wafers in the cassette, and contaminants getting on the wafer from breakdown of the transport mechanism.
The present invention relates to handling of silicon wafers and in particular to edge grip handling of silicon wafers.
A stable edge grip on a wafer can be maintained if there are at least three points of contact between the gripper and the wafer. The force imparted by the three points of contact must be sufficient to counterbalance the weight of the wafer, but must not be so large as to bend the wafer significantly. At least three edge gripping fingers are mounted on a paddle such that the entire paddle/finger assembly can fit between wafers in a cassette. At least two of the edge gripping fingers are fixed at one end of the paddle in a separated relationship and passively respond to pressure brought to bear on them by the wafer. A single finger is mounted on the other end of the paddle and exhibits linear motion. The arrangement of the fingers on opposite sides of the wafer leaves the edges along the axis of the paddle arm free. The movable finger is brought into contact with the wafer by a flexure hinge driven by compressible fluid. Because there is no rubbing motion in the mechanism of the flexure hinge, the mechanism does not wear, does not have backlash or creep-like error motions, and no particulate contaminants are generated. In addition, the use of a compressible fluid mechanism assures that a smooth steady motion is imparted to the moving finger.
In picking up a wafer, the paddle/finger assembly is slid beneath the target wafer until the two fixed fingers are just past the edge of the wafer. The assembly is then raised so that the wafer and gripping surface of all the fingers are approximately aligned. The moving finger then moves toward the wafer until it exerts sufficient force on the wafer to bring the far side of the wafer in contact with the fixed fingers. The moving finger is maintained in the contact position throughout the withdrawal from the cassette and during any transport.
The release of the wafer is accomplished by reversing the steps of pickup. The moving finger breaks contact with the wafer, releasing the force on the wafer. A flexure tip on the fixed fingers nudges the wafer away from edge gripping surfaces of the fixed fingers. Then the paddle/finger assembly is lowered and withdrawn from the proximity of the wafer.